Japanese Patent Unexamined Publication No. 11-201835 discloses a piezoelectric composition pressure sensitive substance having an excellent piezoelectric property and flexibility. This piezoelectric composition pressure sensitive substance is made by mixing and kneading materials including a titanium coupling agent, a piezoelectric ceramic powder, and at least one of chlorinated polyethylene and chlorosulfonated polyethylene. The kneaded materials have flexibility because they contain a thermoplastic elastomer such as chlorinated polyethylene, so that they are processed into a sheet or cable form. The thus processed formed product is a piezoelectric composition pressure sensitive substance to which a pair of electrodes are connected so as to be used as a piezoelectric element.
However, when such a piezoelectric element is used in a high-temperature and high-humidity environment, water vapor passes through the electrodes and reaches the piezoelectric composition pressure sensitive substance. When a temperature of the inside of the piezoelectric composition pressure sensitive substance is lower than that of the outside or when water vapor becomes in a supersaturation state on the surface of the piezoelectric composition pressure sensitive substance, diffusing water vapor is condensed to generate water. Components of piezoelectric ceramic powders constituting the piezoelectric composition pressure sensitive substance or impurities are thus eluted by water. As a result, the electric property of the piezoelectric element is changed, for example, the electrostatic capacitance is increased or the electric resistance is reduced. Such changes lower the durability of the piezoelectric element. Furthermore, when the piezoelectric element is used as a sensor, a circuit for detection or control becomes complicated.
Furthermore, when the piezoelectric element absorbs water, the piezoelectric composition pressure sensitive substance itself is softened and the relation between the pressure and the distortion of the piezoelectric element is changed. Therefore, a signal processing in accordance with the change in the softness of the piezoelectric element is required, and a complicated detection circuit is required because detection error may occur.
Furthermore, since the softening of the piezoelectric element lowers the tensile strength of the piezoelectric element, the durability is deteriorated. In particular, since piezoelectric ceramic powders containing alkali metals of Group I and alkaline earth metals of Group II of the Periodic Table have a small specific resistance and are easily eluted in water, the electric resistance is remarkably lowered.